Wood Internal Stress at Different Drying Stages - Middle Drying Stage
At this stage, the moisture content inside the wood is already below the fiber saturation point. If the spray steaming treatment was not carried out in the previous stage, the outer wood would have lost its normal drying shrinkage condition and was fixed in a stretched state. Just like the incomplete dry shrinkage before the plasticization and fixing of the outer wood, the internal size and the outer layer are temporarily balanced, so the internal stress of the wood is also temporarily in a balanced state. At this time, if the test piece is sawed into a comb shape, the length of each tooth is temporarily the same, but after being placed, the moisture content decreases, and some teeth in the inner layer of the test piece will become shorter due to drying shrinkage.
If the test piece is cut into two pieces, the two pieces will remain straight at that time, but after the moisture content is reduced and distributed evenly, the wood on the inner side will become shorter due to drying shrinkage, causing the two pieces to bend inward. This shows that although the stress in the wood cannot be observed temporarily at this stage, the residual stress in the wood will still appear after drying.
During this stage, the moisture inside the wood has a longer distance to travel to the surface, making it more difficult and slower to dry the wood.
If the outer layer dries too quickly, the internal moisture cannot move to the surface layer in time, resulting in a so-called "wet core" that is very dry on the outside and very wet on the inside. Shell, which not only makes it difficult for moisture inside the wood to enter the air through the surface of the wood, but also affects the shrinkage of the internal wood. This phenomenon is called "surface hardening". If the surface hardening is not removed in time, the drying will be difficult to continue and will lead to serious drying defects. Therefore, spray steaming treatment must be used in this drying stage, and the plasticized and fixed wood surface will be re-moisture-absorbed and softened with high-temperature and high-humidity air to remove it. Surface hardening.
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